1. Field of the Invention
The present invention relates to a motorcycle having a four-stroke engine and a belt type continuously variable transmission.
2. Description of Related Art
JP-B-61-040864 discloses a motorcycle having an engine unit that comprises a two-stroke engine arranged rearwardly of a down frame extending vertically with an axis of a cylinder extending vertically. A belt type continuously variable transmission (“CVT”) is arranged in a crank case, which supports a crank shaft and an output shaft. The output shaft of the engine unit is connected to a rear wheel through a power transmission mechanism such as belt, chain or drive shaft to transmit motive power to the rear wheel.
The motorcycle of JP-B-61-040864 assumes the same configuration as that of an ordinary motorcycle but does not need a gearshift and is therefore high in convenience. Replacement of the two stroke engine of JP-B-61-040864 with a four-stroke engine, which has a relatively large torque in low speed, would further improve convenience. However, replacement of the two-stroke engine of JP-B-61-040864 with a four-stroke engine is problematic.
In a four-stroke engine, intake valves, exhaust valves and a valve operating mechanism for driving the intake and exhaust valves must be arranged on a cylinder head. Also, an oil reservoir portion must be formed below the crank shaft to store lubricating oil. Therefore, a four-stroke engine is relatively long in an axial direction of a cylinder relative to a two-stroke engine. In trying to apply such four-stroke engine to a motorcycle, in which the engine is arranged rearwardly of a down frame extending vertically with axes of cylinders extending vertically, the motorcycle cannot but be increased in total height, or decreased in minimum ground level, since the engine is long in an axial direction of the cylinders. Accordingly, it is difficult from a compatibility standpoint to simply replace the two-stroke engine of the motorcycle of JP-B-61-040864 with a four-stroke engine.